A phase change memory (PRAM) is a device in which data is stored by controlling crystalline state of a phase change material such as Ge2Sb2Te5 with Joule's heat generated by current flowing through the phase change material.
If the device is heated to higher than its melting point, and cooled at a faster rate, the device is turned into a high-resistance amorphous state (RESET state).
If the phase change memory is heated to a temperature not lower than its melting point, kept at this temperature for a longer time and cooled at a slower rate, the device is in a low-resistance crystallized state (SET state). Data is stored in the phase change memory by controlling the two types of crystalline states, that is, the high-resistance state and the low-resistance state, with write pulses applied thereto.
The time required for writing, in particular for crystallization, depends on the time of crystallization of the material used. Hence, data writing needs some time duration. The write time needed is said to be tens to hundreds of ns (nano-seconds) with a material preferentially used as a commonly used phase change material, such as Ge2 Sb2Te5.
As regards the phase change memory (PRAM), reference is made to, for example, the disclosure of Patent Document 1.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2005-100617A